Ribbed conveyor belts are old in the art as disclosed in U.S. Pat. Nos. 3,070,210; 2,815,850; 2,784,835; and 2,400,667 and the provision of a ribbed conveyor belt with planar portions along its opposite longitudinal edges for engagement beneath the skirt boards on the conveyor supporting frame is shown in Pat. No. 2,880,128. In that patent, the lower edges of the skirt rest upon the surfaces of the planar portions at the opposite longitudinal edges of the belt in the same plane as the surfaces of the planar areas between the ribbed center portion of the belt, and since the skirting is of a flexible material and is subjected to uneven of the particulate material being carried along by the belt, and varying degrees of wear and wetness or drying, its lower edge which has contact with the planar surfaces may lift away from the planar surfaces so that the particulate material can escape at the sides of the conveyor belt. The purpose of this invention is to minimize such deflection of the lower edges of the skirts and to thus minimize the escape of particulate material.